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The first submarine to down an enemy ship was sunk itself after its crew failed to release an emergency weight to help it resurface. 

Crew aboard the Confederate vessel HL Hunley did not disconnect the 1,000lb keel blocks to help it rapidly resurface, resulting in the sub being trapped underwater and the men dying from lack of oxygen.

Scientists who removed the corrosion, silt and shells from the boat found the levers all locked in their regular position, solving a mystery dating back to 1864.  

The blocks would typically keep the sub upright, but also could be released with three levers. That would allow it to surface rapidly, archaeologist Michael Scafuri, who has worked on the submarine for 18 years, said. 

'It's more evidence there wasn't much of a panic on board,' Scafuri said.

The Hunley and its eight crewmembers disappeared in February 1864 in Charleston Harbor shortly after signaling it had placed explosives on the hull of the Union ship the USS Housatonic. 

Confederate submarine H.L. Hunley sits in a conservation tank at a lab in North Charleston, South Carolina 

In October 1863, designer H.L. Hunley led another eight-man crew who planned to show how the sub (pictured) operated by diving under a ship in Charleston Harbor

One of the two preserved keel blocks that were removed from the Hunley submarine. The doomed Confederate crew did not release an emergency mechanism that could have helped the vessel surface quickly, scientists said 

Housatonic (pictured) lost five seamen, but came to rest upright in 30 feet of water, which allowed the remaining crew to be rescued after climbing the rigging and deploying lifeboats

The Hunley laying in a preservation tank. After sinking the USS Housatonic during the Civil War, the Hunley, and its' crew disappeared

The Hunley's torpedo was not a self-propelled bomb. Rather, it was a copper keg of gunpowder held ahead and slightly below the Hunley's bow on a 16-foot pole called a spar (pictured)

James McClintock (left and right) was one of the designers of the doomed Confederate submarine H.L. Hunley (1863-1864)

A schematic of the submarine from 1863 shows the rudimentary system of propulsion via levers. The men are shown cramped inside the vessel 

The Hunley had delivered a blast from 135 pounds of black powder below the waterline at the stern of the Housatonic, sinking the Union ship in less than five minutes.

Housatonic lost five seamen, but came to rest upright in 30 feet of water, which allowed the remaining crew to be rescued after climbing the rigging and deploying lifeboats.

Ever since the Hunley was raised from the ocean floor in 2000, scientists have worked to determine why the sub never returned to the surface. 

The keel blocks don't give a definitive answer, but do provide clues that either the crew didn't think it needed to surface quickly or never realized they were in danger.

The crew moved the submarine through the ocean with a hand crank, and one theory is they were resting on the ocean floor 4 miles from shore waiting for the tide to turn to make their journey back to land easier and ran out of oxygen or got stuck.

But there are other theories, such as the Housatonic explosion knocking out the Hunley's crew or a ship that sped to help save some of the crew on the Union ship clipping the Confederate sub and crippling it as it tried to dive. 

Senior conservator Paul Mardikian (pictured in 2012) wets down the Civil War submarine H.L. Hunley after it was freed of the steel truss that was used to raise it from the ocean floor in 2000

Conservator Anna Funke sprays sodium hydroxide on the H.L. Hunley in the Warren Lasch Conservation Center in North Charleston in June last year 

Conservator Virginie Ternisien works at removing the encrustation from the hull of the Hunley in 2015

Those theories can't be ruled out - at least not yet and maybe never, said Scafuri, who planned to work on the Hunley mystery for a year or two as a graduate student in 2000 and is now entering his 18th year helping conserve and study the submarine which is stored in chilled, fresh water in a 75,000-gallon tank in North Charleston.

Over 18 years, Scafuri said they have uncovered nearly a dozen artifacts, reconstructed the faces of the crew members and gained more knowledge about the science behind the submarine, which was built in Mobile, Alabama.

'We keep seeing parts that no one has seen in 150 years. All of them add into the mix of what happened and how this sub was operated,' Scafuri said. 'After all, we don't have the blueprints.'

The keel blocks go on display at the Hunley's North Charleston museum Saturday. 

Workers lifting a keel block from the Hunley submarine, out of a 75,000 gallon conservation tank at the Warren Lasch Conservation Center

A Clemson University conservator attempting to separate the keel block from the Hunley at the Warren Lash Conservation Center in North Charleston, South Carolina

The submarine sank once while docked with its hatches open in August 1863. Only three of the eight men on board escaped and survived. Pictured: The wreck in 2012

'I would love to get to that point absolutely,' Scafuri said when asked if he thinks scientists will ever know exactly what happened inside the sub, which was just 40 feet long and so small the men couldn't stand up straight as they turned the crankshaft.

'Can I promise that? No,' Scafuri said.

The next step for scientists is to remove more of the corrosion, slit and other material collected on the hull.  

Fascinatingly, the Hunley's successful but doomed final mission was actually its third trip. 

The submarine sank once while docked with its hatches open in August 1863. Only three of the eight men on board escaped and survived.

In October 1863, designer H.L. Hunley led another eight-man crew who planned to show how the sub operated by diving under a ship in Charleston Harbor.

They never surfaced, but the sub was found weeks later and brought back to the surface. 

That crew was interred in graves that ended up below The Citadel's football stadium for 50 years. 

The narrow, top-secret 'torpedo fish' was built with cast and wrought iron with a hand-cranked propeller and arrived in Charleston in 1863 while the city was under siege by Union troops and ships.

The Confederate Navy hauled the sub up twice, recovered the bodies of the crew, and planned a winter attack.  

Before the explosion, a lookout on the Housatonic spotted a bizarre vessel approaching just below the surface - with only its coning tower visible - and sounded the alarm.

The Housatonic's cannons couldn't be lowered enough to fire at the strange craft, so crewmen used rifles and pistols, but these were not effective.

Some historians say that the submarine showed a mission-accomplished lantern signal from its hatch to troops back on shore before it disappeared.

Soon after the signal had been fired, the sub sank about 4 miles off Charleston.

Ever since the Hunley was raised from the ocean floor in 2000, scientists have worked to determine why the sub never returned to the surface

The sub itself is only four feet in diameter. Eight schoolchildren can barely cram themselves into a replica nearby at the Warren Lasch Conservation Centre

Park and Lyons machine shop building, Mobile, where the Confederate submarine H.L. Hunley was constructed in 1863. Located at the corner of Water and State Streets, in Mobile, this old building housed the Gill Welding and Boiler Works when photographed in about 1960

Inboard profile and plan drawings, after sketches by W.A. Alexander, who directed the submarine's construction

More than a century later, in 1995, the Hunley was discovered off the South Carolina. It was raised in 2000 and brought to a conservation lab in North Charleston.   

The crewmen's skeletons were found still at their stations along a hand-crank that drove the cigar-shaped craft.

They suffered no broken bones, the bilge pumps hadn't been used and the air hatches were closed.

Except for a hole in one conning tower and a small window that may have been broken, the sub was remarkably intact.

The eight crew members were buried in an elaborate ceremony at a Confederate cemetery in Charleston in 2004.

They were the sub's commander, Lt George Dixon of Alabama, James A Wicks, a North Carolina native living in Florida, Frank Collins of Virginia, Joseph Ridgaway of Maryland and four foreign-born men about whom less is known.

One is still only known as 'Miller.'

The sub itself is only four feet in diameter. Eight schoolchildren can barely cram themselves into a replica nearby at the Warren Lasch Conservation Centre.